This mod allows you to create schematics with goxel.
Download Goxel: https://goxel.xyz/download/
First, you need to create a palette for your world that maps colors to nodes.
Create a world, enable the mod, and send /save_palette
Locate the .gpl palette in the the newly created /worlds/<world_name>/goxel_tools/world_name.gpl
copy the palette to the goxel palettes folder (open Goxel, Edit > Settings > Paths, it will show you your palettes path)
Restart goxel
In Goxel, go to Palette and select the drop down to find your world_name palette. Node names are arranged alphabetically.
Build your model.
When done, select File > Export > text
Save the text file to the world folder into goxel_tools
Open Luanti to your world, and run /place_goxel file_name or /place_goxel file_name X Y Z
(not /place_goxel file_name.txt) to place the model.